A battery (battery block) including a number of cells connected with each other in a given arrangement is mounted on a vehicle. Housing the battery in a case forms a battery module. To prepare for use in cold climate, the battery module incorporates a battery heater device which heats the battery to a preset temperature.
It is desirable to heat the respective cells of the battery to a uniform temperature with use of the battery heater device. In order to heat the cells uniformly, it is proposed to adjust densities of heater wires in the battery heater device so as to change amount of heat generated by the heater wires depending on its position (for example, see Patent Document 1).